1. Field of the Invention
The present invention relates to an optical interferometer to measure a distance and a position of a target.
2. Description of the Related Art
Conventionally, the method of measuring the distance by the optical interferometer is well-known. A distance measurement by the optical interferometer uses a signal of interference light (interference signal) generated by an interference between a reference beam which is reflected on a reference surface being the distance measurement reference and a test beam which is reflected on a target surface attached to a target, and thereby using a wavelength of the light as the scale. Since the wavelength changes by a refractive index of an optical path of the test beam, a high-accurate correction of the refractive index is essential in order to achieve a high-accurate length measurement in an atmosphere.
Japanese Patent Laid-Open No. H10-281717 discloses a method of correcting a refractive index with high accuracy in a general environment, namely environment which has a spatial distribution of atmosphere density. This method is well known as the two-color method because the refractive index and the distance is similarly measured based on optical path length of two wavelengths or more by using the refractive index dispersion of the atmosphere.
U.S. Pat. No. 6,330,065, in the specification, discloses a method of achieving a high-accurate correction of the refractive index by arranging an interferometer for dispersion measurement and calculating the dispersion of a measurement optical path of a distance measurement interferometer using the measurement result of the interferometer for dispersion measurement.
In the two-color method of Japanese Patent Laid-Open No. H10-281717, the refractive index dispersion is calculated by a refractive index expression such as Edlen equation. However, since the accuracy of the refractive index expression is no more than about 10−8, the method lacks the accuracy for the high-accurate correction, and causes an error of the refractive index dispersion in air compositions in the length measurement environment.
Moreover, in the method described in U.S. Pat. No. 6,330,065, when the atmosphere composition of the optical path measured by the interferometer for dispersion measurement is different from that of the optical path measured by the interferometer for distance measurement, the air dispersion has the difference between the interferometer for dispersion measurement and the interferometer for distance measurement. Therefore, the correction accuracy of the refractive index decreases.